Clinical experience over the last two decades has shown that antiplatelet--therapy is often not effective in the treatment of arterial thrombotic disorders. Patients on aspirin and other agents which impair platelet cyclooxygenase and the platelet release reaction continue to suffer recurrent episodes of myocardial ischemia or recurrent stroke. We therefore hypothesize that some arterial thrombotic disorders are in part due to the procoagulant activity (PCA) of a stimulated, inflammed or injured vessel wall. This view is in keeping with recent in vitro data that vascular endothelium, when stimulated by inflammatory/immune mediators such as interleukin-l (IL-l), tissue necrosis factor (TNF), lymphotoxin or hepatic stimulating factor, can become prothrombotic by generating, for example, tissue factor and molecules adhesive for granulocytes. Simultaneously, vasculitis and/or vessel wall injury may diminish the production of substances of arachidonate (prostacyclin and PGE2) and non-arachidonate (endothelial cell relaxing factor) metabolism which normally modulate leukocyte-platelet adhesion/aggregation. The experimental approach taken here to evaluate this hypothesis is to simulate in vitro flow through a microvessel. We do so by utilizing a flow chamber which incorporates whole blood, arterial-like shear rates, a flow path of thickness 290 um, and in situ epifluorescence video- microscopy of leukocyte-platelet adhesion/aggregation to a defined injury site on an endothelial cell monolayer. Our aims include 1) establishing the effects of flow (shear rate and shear stress) on PCA; 2) determining the dependence on flow of granulocyte adhesion to non-injured endothelium; and 3) determining the flow dependence of leukocyte-platelet adhesion/aggregation to a site of defined injury to endothelium. PCA will be measured with flowing, cell- free media and a radiometric assay for Factor Xa conversion. For imaging purposes,platelets will be labelled with a fluorescein- conjugated monoclonal antibody (TAB) to glycoprotein IIB, while monocyte-granulocytes will be labelled with a rhodamine-conjugated monoclonal antibody directed against the Mo5 antigen. In particular, we seek a rational basis for the use of anticoagulant therapy in certain cases of arterial thrombosis.